Here's a little TLDR for those interested to cover the whole 5 pages:
The most important clue:
-Over the course of the last week of it running, it slowly lost power each time i drove it. Eventually the last day, it would go over 30mph, so i started to head back home just in case, and it wouldn't go over 15. Eventually i tried to make it up a hill, and the truck just stopped halfway up. The engine didn't shut off (automatic,) it just didn't make enough power to get over idle on this hill. I got home crawling at 5mph and it hasn't started since.
-Everything between and including the sending unit and filter head has been replaced. (Had cracked fuel lines, and a rotted sending unit so i just did everything)
-FSS confirmed working, and still works while cranking
-Batteries fully charged and charger plugged in for cranking. The engine spins over faster now than it ever did when it was in its running state, so I'm gonna rule out the starter.
-Replaced injector return lines with viton o-rings
-ELECTRIC fuel pump putting out 7 psi and 1/3 pint after 10 seconds
-Getting fuel out of injector lines, but idk if it's enough or timed right.
-Smokes very little when cranking over, usually it smokes quite a bit
-Glow plugs confirmed working, won't start with glow plugs or with ether (never used both at the same time for obvious reasons)
-Getting fuel to injection pump
I'm hoping it's the IP so I can just get it over with. Is there any way to test it without it running? By test i mean be absolutely certain that it's bad before i spend the money on it. I've read different methods for hot starts and while it's running, but not a definitive test without it running. Thanks for all y'all's help, it's much appreciated